Polymethylmethacrylate (PMMA) bone cements are based on the pioneering work of Sir Charnley. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component, which is also referred to as bone cement powder, comprises one or more polymers, which are produced on the basis of methylmethacrylate and comonomers, such as styrene, methylacrylate or similar monomers by means of polymerisation, preferably by suspension polymerisation. The bone cement powder additionally comprises a radiopaquer and the initiator dibenzoylperoxide. During the mixing of powder component and monomer component, swelling of the polymers of the powder component in the methylmethacrylate generates a dough that can be shaped plastically and is the actual bone cement. During the mixing of powder component and monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Methylmethacrylate is the monomer used most commonly in polymethylmethacrylate bone cements. Redox initiator systems usually consist of peroxides, accelerators and, if applicable, suitable reducing agents. Radicals are formed only if all ingredients of the redox initiator systems interact. For this reason, the ingredients of the redox initiator system in the separate starting components are arranged appropriately such that these cannot trigger a radical polymerisation. The starting components are stable during storage provided their composition is adequate. Only when the two starting components are mixed to produce a cement dough do the ingredients of the redox initiator system, previously stored separately in the two pastes, liquids or powders, react with each other, forming radicals which trigger the radical polymerisation of the at least one monomer. The radical polymerisation then leads to the formation of polymers while consuming the monomer, whereby the cement dough is cured.
PMMA bone cements can be mixed by mixing the cement powder and the monomer liquid in suitable mixing beakers with the aid of spatulas. One disadvantage of said procedure is that air inclusions may be present in the cement dough thus formed and can cause destabilisation of the bone cement later on.
In order to avoid air inclusions in the bone cement dough, a large number of vacuum cementing systems have been disclosed of which the following shall be listed for exemplary purposes: U.S. Pat. No. 6,033,105 A, U.S. Pat. No. 5,624,184 A, U.S. Pat. No. 4,671,263 A, U.S. Pat. No. 4,973,168 A, U.S. Pat. No. 5,100,241 A, WO 99/67015 A1, EP 1 020 167 A2, U.S. Pat. No. 5,586,821 A, EP 1 016 452 A2, DE 36 40 279 A1, WO 94/26403 A1, WO 00/35506 A1, EP 1 005 901 A2, U.S. Pat. No. 5,344,232 A. In the vacuum cementing systems thus specified, there is a need to connect an external vacuum pump to generate the negative pressure. These are generally operated by compressed air utilising the Venturi principle. The compressed air required for operation of the vacuum pumps is supplied either by stationary compressed air facilities or by electrically-operated compressors. In addition, it is also feasible to use electrically-operated vacuum pumps to generate vacuum.
Cementing systems in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement, are a development of cementing technology. Such full-prepacked mixing systems were proposed through EP 0 692 229 A1, DE 10 2009 031 178 B3, U.S. Pat. No. 5,997,544 A, U.S. Pat. No. 6,709,149 B1, DE 698 12 726 T2 and U.S. Pat. No. 5,588,745 A. Patent DE 10 2009 031 178 B3 discloses a vacuum mixing device having a two-part dispensing plunger that can also be used for a mixing device according to the invention. Here, a combination of a gas-permeable sterilisation plunger and a gas-impermeable sealing plunger is used.
Document WO 00/35506 A1 proposes a device in which PMMA bone cement powder is stored in a cartridge, wherein the cement powder fills the entire volume of the cartridge and the gaps between the particles of the cement powder are of a volume corresponding to the volume of the monomer liquid necessary for the production of bone cement dough together with the cement powder stored in the cartridge. This device is constructed such that, as a result of the effect of a vacuum, the monomer liquid is introduced form above into the cartridge, wherein for this purpose a vacuum is applied at a vacuum connection at the underside of the cartridge. The monomer liquid is thus drawn through the cement powder, wherein the air disposed in the gaps between the cement particles is displaced by the monomer liquid. Here, there is no mechanical mixing of the formed cement dough by means of an agitator.
A disadvantage of this system is that cement powders, which swell quickly when exposed to the monomer liquid, cannot be mixed with this device, because the rapidly swelling cement powder particles, once the monomer liquid has penetrated the cement powder by approximately 1 to 2 cm, form a gel-like barrier and prevent the migration of the monomer liquid through the entire cement powder. In addition, under the influence of a vacuum, it cannot be ruled out that, after complete penetration of the cement powder by the monomer liquid, the monomer liquid might be sucked up via the vacuum connection. There then would be insufficient monomer liquid available for curing by radical polymerization, or the mixing ratio might be modified unintentionally, and therefore also the consistency of the bone cement. It is also problematic that the air enclosed between the cement powder particles will be displaced from top to bottom by the monomer liquid, because the air, which specifically is lighter then the monomer liquid, will seek to move upwardly in the cement powder on account of the effect of gravity rather than migrating downwardly in the direction of the vacuum connection.
If mixing devices are used for cementing, external vacuum pumps need to be provided. Said vacuum pumps are expensive and need to be cleaned after use. Moreover, vacuum hoses for connecting the vacuum pumps to the mixing devices are required. Said vacuum hoses need to be enclosed with the mixing devices. Accordingly, prior to the mixing using a mixing device, the vacuum pump needs first to be set up in the operating theatre (OP theatre) and must be connected to an energy source, such as compressed air, or to an electrical power source. Then, the vacuum pump is connected to the mixing device by means of a vacuum hose. Said installation steps take up costly OP time and are potentially error-prone. The vacuum pump and connecting lines to the mixing device and to external energy sources and supply lines take up space and constitute potential tripping hazards and obstacles that can disturb the often hectic procedure during surgery.
A further interesting concept has been proposed through EP 1 886 647 A1. Here, the cement powder is stored in an evacuated cartridge and the monomer liquid is situated in a separate container. The cartridge, which is kept at a negative pressure, being opened causes the monomer liquid to be aspirated into the cartridge without any ingress of air. A bone cement dough free of air inclusions is thus produced. Said concept requires the cartridge to remain closed in vacuum-tight manner during the storage before use such that no non-sterile air can enter into the cartridge. For this purpose, the cartridge must be sealed in a stable hermetic manner. Accordingly, one associated disadvantage is that the design is quite elaborate and that the content in the cartridge cannot be mixed by an externally-operated mixing system after aspiration of the monomer since a feedthrough for a mixing rod or for a mixing tube would not readily be permanently vacuum-tight. In the case of known full-prepacked mixing systems, a vacuum or a negative pressure is usually used to transfer the monomer liquid into the cement powder.